Appendix A
GPIB Basics
©
National Instruments Corporation
A-3
GPIB-120B User Manual
Controller-In-Charge and System Controller
You can have multiple Controllers on the GPIB, but only one Controller at
a time can be the active Controller, or Controller-In-Charge (CIC). The CIC
can be either active or inactive (standby). Control can pass from the current
CIC to an idle Controller, but only the System Controller, usually a GPIB
interface, can make itself the CIC.
GPIB Signals and Lines
Devices on the bus communicate by sending messages. Signals and lines
transfer these messages across the GPIB interface, which consists of
16 signal lines and 8 ground return (shield drain) lines. The 16 signal lines
are discussed in the following sections.
Data Lines
Eight data lines, DIO1 through DIO8, carry both data and command
messages.
Handshake Lines
Three hardware handshake lines asynchronously control the transfer of
message bytes between devices. This process is a three-wire interlocked
handshake, and it guarantees that devices send and receive message bytes
on the data lines without transmission error. Table A-1 summarizes the
GPIB handshake lines.
Table A-1.
GPIB Handshake Lines
Line
Description
NRFD
(not ready for data)
Listening device is ready/not ready to receive a
message byte. Also used by the Talker to signal
high-speed GPIB transfers.
NDAC
(not data accepted)
Listening device has/has not accepted a message
byte.
DAV
(data valid)
Talking device indicates signals on data lines are
stable (valid) data.
